Nested functions

Functions can be defined inside other functions:

const dosomething = () => {
  const dosomethingelse = () => {
    //some code here
  }
  dosomethingelse()
  return 'test'
}

The nested function is scoped to the outside function, and cannot be called from the outside.

This means that dosomethingelse() is not reachable from outside dosomething():

const dosomething = () => {
  const dosomethingelse = () => {
    //some code here
  }
  dosomethingelse()
  return 'test'
}

dosomethingelse() //ReferenceError: dosomethingelse is not defined

This is very useful because we can create encapsulated code that is limited in its scope by the outer function it’s defined in.

We could have 2 function define a function with the same name, inside them:

const bark = () => {
  const dosomethingelse = () => {
    //some code here
  }
  dosomethingelse()
  return 'test'
}


const sleep = () => {
  const dosomethingelse = () => {
    //some code here
  }
  dosomethingelse()
  return 'test'
}

and most importantly you don’t have to think about overwriting existing functions and variables defined inside other functions.

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